Could be wrong here but I'm pretty sure the membranes on a bat's wings are exceptionally thin, I doubt they'd provide much protection at all. Talking out of my arse here but I'd assume that making anything out of rabbit hide would almost certainly be a far better option than using bat wings. As a side note aren't bats known as vectors for diseases? Are we sure it's such a good idea to be hanging around them? I guess goblins are just more resistant to disease than people.